As a method for simultaneously performing a liquid-phase epitaxial growth on the surfaces of a plurality of crystalline substrates, there has been known a process wherein a plurality of crystalline substrates are received in a holder in such a manner that they are arranged in a state faced to each other with predetermined intervals along an approximately vertical direction, the gaps between the crystalline substrates are filled with a melt for crystal growth maintained in a saturated state at a high temperature or in a supersaturated state by slightly lowering the temperature, and the melt is held in contact with the surface of each crystalline substrate to deposit an epitaxial layer on the surface of each crystalline substrate by proper temperature control such as slow cooling.
In a liquid-phase epitaxial crystal growth process, it is comparatively easy to epitaxially grow only one layer on the surface of each crystalline substrate. As the most principal means for this purpose, there have been developed various methods so far, wherein an alignment of the substrates is lowered and dipped into a melt maintained at a lower position with respect to the alignment of the crystalline substrates, to bring out crystal growth on the surface of each substrate, and then the alignment of the substrates are raised and separated from the melt.
There is, for example, a known method wherein an alignment of crystalline substrates are supported with an eccentric rotary shaft in a manner such that the crystalline substrates are dipped in the melt and then separated from the melt using the rotation of the eccentric rotary shaft. In another method, a melt is pumped by a piston and brought into contact with the alignment of crystalline substrates located at a higher position.
In these known methods, however, the melt is circulated relatively upwards to the gaps between adjacent crystalline substrates, and then let flow down after the growth of a proper crystalline layer. But, since the oxide films and microcrystals deposited from the melt are floating on the surface of the melt or suspending in the upper layer of the melt, the oxide films and/or microcrystals are apt to adhere onto the surface of an grown layer during dipping and lifting the substrate. The adhesion of the oxide films and/or microcrystals, i.e. the contaminated surface of the epitaxial layer, causes troubles on the growth of a normal crystal layer thereon, especially when the growth of a second or more layers are performed in the succeeding steps.
When the next crystal growth for the second layer is performed by further bringing the first epitaxial layer contaminated with the oxide films and/or the microcrystals into contact with another melt for the second layer, there will occur stacking faults originated in the part where the oxide films and/or the microcrystals adhered. In the case where the oxide films and/or the microcrystals exhibit significant influences, said part becomes to an upgrowth pit. Epitaxial layers involving such defects are not proper for the formation of electronic devices. Even if the electronic devices are formed using the said materials, the obtained products would exhibit very low performances and lack in reliabilities.
The influence of the defects will become larger when the third or more layers are further piled up in a multi-layered state. The uppermost layer substantially lacks in flatness, hence the multi-layered crystals can not be handled just for the process of producing electronic devices.
As above-mentioned, the influence of oxide films and/or microcrystals floating on the surface of a melt is inevitable in the liquid-phase epitaxy method, as far as the melt is circulated relatively upwards into the gaps between adjacent crystalline substrates and then let flow down after the growth of a proper crystalline layer.
In order to eliminate the aforementioned defects, there is known another method as shown in FIG. 1, wherein a melt is supplied from above the alignment of crystalline substrates.
With reference to FIG. 1, a holder b for receiving the alignment of crystalline substrates is provided in a horizontal tubular electric resistance furnace a. A reservoir c for reserving a melt to be used for crystal growth is located above the holder b, while a reservoir d for receiving the melt used for the crystal grow this located below the holder b. The reservoirs c and d are faced to each other along a vertical direction, and an upper shutter blade e and the lower shutter blade f are disposed between the reservoirs c and d, the upper shutter blade e and lower shutter blade being in a state each capable of sliding along the axial direction of the furnace a. The upper shutter blade e has a circulation hole h formed through its thickness to let a melt g flow down through the upper shutter blade e.
When the circulation hole h of the upper shutter blade e is brought to the position corresponding to a hole formed in the bottom wall of the reservoir c by the movement of the upper shutter blade e, the melt g is let to flow down from the reservoir c and poured in the holder b. Hereon, only the lower layer of the melt g in the reservoir c flows down to the holder b, while the upper layer of the melt g contaminated with formed oxide films and/or deposited microcrystals remains as such in the reservoir c. Consequently, the holder b is filled with the pure melt g free from inclusions. The alignment i of crystalline substrates in the holder b comes in contact with the uncontaminated pure melt g, so that a proper epitaxial layer grows on the surface of each substrate.
After the completion of the epitaxial growth, the lower shutter blade f is carried to the position where a circulation hole j formed in the lower shutter blade f comes to the position corresponding to a discharge hole formed in the bottom wall of the holder b. The used melt g in the holder b flows down to the reservoir d, and the alignment of the crystalline substrate is released from the condition in contact with the melt g.
The shutter blades e and f are individually carried along the axial direction of the furnace a by operating rods k. The reservoir c for the melt g to be used for crystal growth is carried along the axial direction inside the furnace a by another operating rod l. A reaction zone for the crystal growth is isolated from the outside atmosphere by a quartz tube m.
When the melt g for the crystal growth is let flow down, brought into contact with the alignment i of crystalline substrates and then discharged downwards to the used melt reservoir d as above-mentioned, a contaminated melt containing oxide films and/or microcrystals is prevented from flowing into the holder b. Therefore, the formation of crystallographic defects derived from the oxide films and/or the microcrystals can be inhibited.
In order to apply this method to a process for the production of multi-layered crystals, it is necessary to provide a plurality of reservoirs c for reserving various kinds of melts above the holder b receiving the alignment i of crystalline substrates therein in the order of crystal growth steps. Hereon, in a commonly adopted manner, the holder b is stationarily held at an approximately central position with respect to the axial direction of the furnace a, while reservoirs c for receiving a plurality of melts are slidingly disposed right-side and/or left-side above the holder b. Each melt is different in the kind of a dopant, concentration, and composition for determining the mixing ratio of a deposited layer from the other. The reservoirs c are intermittently carried to a position above the holder b in the order of crystal growth steps.
The reservoirs c for the melts to be used for crystal growth are arranged in series above the horizontal tubular electric resistance furnace a to intermittently pour the melts to the holder b. This arrangement requires an extremely long soaking zone for maintaining the melt reservoirs c at predetermined temperatures. In general, a furnace body has a total length proportional to the length of the soaking zone. In this regard, a space occupied by the furnace becomes huge, and the other large space for auxiliary facilities such as a clean room is also required. Due to the higher cost of equipment, it is obliged to manufacture products such as epitaxial wafers at higher manufacturing costs.
If a plurality of the melt reservoirs c are to be held without making the soaking zone longer, each melt reservoir c shall have a deep cavity for reserving a melt in an amount necessary for epitaxy. In this case, there is required an electric resistance furnace a having a large inner diameter enough to locate such deep reservoirs c therein. The enlargement of the inner diameter causes the deterioration of a soaking condition along the radial direction of the furnace a, hence appropriate conditions for proper epitaxial crystal growth can not be obtained.
In the liquid-phase epitaxy method for multi-layered crystals, each melt used for crystal growth is let flow down and separately recovered in a corresponding used melt reservoir d. The recovered melt, as such or after supplementing some components if necessary, is returned to the holder b and reused for the next crystal growth. For this reuse of the melts, there is required an operating mechanism for intermittently carrying a plurality of used melt reservoirs d to the position below the holder b stationarily located in the furnace a. As a result, inner members to be carried along the axial direction of the furnace a are 4 in total, i.e., the upper shutter blade e, the lower shutter blade f, the melt reservoir c and the used melt reservoir d.
Some of them might be operated in linkage by designing their configurations to simplify the operating mechanisms. Even if a proper linkage system is available for the purpose, it is still necessary to provide at least three operating rods in a manner such that they can be slidingly pulled out from the reactor tube of the furnace body. By the way, the epitaxy process for producing semiconductive materials uses high-purity hydrogen gas in general, so that an apparatus therefor is equipped with very complicated sealing means which is difficult and troublesome to handle. The sealing means often causes troubles or accidents. When mechanisms for pulling the three operating rods are incorporated in the apparatus, three sealing means are also required. As a result, the apparatus is very complicated and hard to operate.
In a liquid-phase epitaxy process using Ga as a main component in a melt for growing an epitaxial layer such as GaAs or Ga.sub.1-x Al.sub.x As, a Ga.sub.2 O.sub.3 film spontaneously formed on the surface of the Ga melt is evaporated off by baking treatment. During the baking treatment, a melt in a state not yet doped with a dopant such as Zn, Cd or Te having a high evaporation pressure is heated for several minutes to several tens of minutes at a temperature above approximately 600.degree. C. The Ga.sub.2 O.sub.3 film is reacted with Ga in the melt and converted into volatile Ga.sub.2 O gas by the baking treatment. The Ga.sub.2 O gas is diffused to a hydrogen stream circulating above the melt and removed from the melt.
When each melt is to be doped with a dopant having a high vapor pressure, the melt is brought into contact with a dopant receiver provided at the upper shutter blade e at the end of the baking treatment. Hereon, if the melt reservoir c is not equipped with capping means, a low-temperature part inside the reactor tube m would be contaminated by the condensation of the evaporated dopant. The condensation would cause the contamination of another melt to be used for the next crystal growth step.
The mutual contamination is preferably inhibited by attaching a cap to the upper opening of each melt reservoir c. However, the attachment of the cap requires either another operating rod for releasing or covering the upper surface of the melt reservoir c and introducing the dopant to the melt, or making the strokes of sliding members longer to enable the opening and closing motion of the cap. Consequently, it is obliged to make the operating mechanisms more complicated or to make the length of the furnace body much longer. In any cases, the production of epitaxial wafers can not be performed at a low cost.
According to the known methods as above-mentioned, three or four operating rods shall be provided in the reactor tube in a manner such that each rod can be pulled out from the reactor tube. In return, the operating mechanism for sliding each member necessary for the epitaxy process is complicated and hard to operate.
In order to avoid the complication of the operating mechanisms, there have been proposed various methods wherein melt reservoirs and used melt reservoirs are stationarily disposed along the axial direction in a horizontal tubular furnace, while a holder for receiving the alignment of crystalline substrates is slidingly provided there. In this case, upper and lower shutter means are attached to the holder.
However, a portion for holding the crystalline substrates shall be maintained under a critically controlled temperature condition. The movement of such a portion along the axial direction of the furnace causes unfavorable fluctuations in the temperature. Especially when a compound semiconductor is to be produced by the liquid-phase epitaxy process, the temperature of the melt in contact with the crystalline substrates shall be controlled with very high accuracy, e.g. within the range of .+-.0.1.degree. C. or less in general.
Such critical temperature control would be impossible, if the holder for receiving the alignment of crystalline substrates is carried along the axial direction. In the method above-mentioned, the holder is repeatedly carried along the axial direction of the furnace at a sliding speed of a few cm/min. or more, hence the temperature control necessary for crystal growth can not be performed with high accuracy. The holder for receiving the alignment of crystalline substrates has a heat capacity and a surface area for heat diffusion different from those of the other members. When the holder is carried along the axial direction of the furnace in a long distance, the heat balance between absorption and diffusion is destroyed through the holder, and the temperature control temporarily comes to a state impossible to maintain the heat balance. Thus, the movement of the holder being most sensitive to the fluctuations in the temperature makes it difficult to simplify operating mechanisms necessary for various operations such as melt supply, epitaxial growth, discharging used melts, separately receiving used melts, closing the opening of the holder with a cap during baking or after doping, and releasing the opening of the holder during doping. As a result, the known epitaxy process using the conventional horizontal tubular furnace requires complicated mechanisms difficult to handle.
There has been also proposed another epitaxy process using a vertical tubular furnace instead of the horizontal type. In some of the known methods using the vertical tubular furnace, a plurality of crystalline substrates are held on the upper surface of a cylinder-type or disc-type holder in a manner such that each substrate is radially placed one by one in a horizontal state or a state slightly inclined from the horizontal plane at a position near the periphery of the holder about its central axis. Some of them are industrially adopted.
However, the number of crystalline substrates to be used for epitaxial growth at the same time is very small, as compared with the methods wherein a plurality of crystalline substrates are vertically arranged in a manner such that the surface of each substrate designed to grow an epitaxial layer thereon is held vertically and faced to the surface of an adjacent substrate with a gap of approximately 2 to 3 mm, and a melt for crystal growth is let flow down through the gaps between the adjacent substrates. In this sense, the known methods using the vertical tubular furnace are different from the method of the present invention.
In short, the known method using the vertical tubular furnace is one modification of a sliding process wherein the crystalline substrates are substantially held in a horizontal state, a melt for crystal growth is let flow horizontally and brought into contact with the surface of each substrate, and the used melt is horizontally circulated after the completion of the crystal growth. In this meaning, the method belongs to a rotary sliding process wherein a sliding direction is determined along a horizontal peripheral direction. According to this sliding process, crystalline substrates and melts are simply combined together in several couples, and the number of crystalline substrates to be arranged on the upper surface of a cylinder-type or disc-type holder is limited to 4 to 8 as for 3-inch wafers unless the tubular furnace has an extremely larger inner diameter.
This sliding process is quite different from the method using the alignment of crystalline substrates to which the present invention is directed, also in the physiological substance of technical contents. According to the sliding process, a melt is supplied to or removed from the surface of a crystalline substrate or the surface of an epitaxial layer previously formed on the crystalline substrate by carrying a melt holder along a direction parallel to the surface of the crystalline substrate under the condition where the melt is held in contact with or nearly contact with the surface of the crystalline substrate. On the other hand, in the method using the alignment of crystalline substrates to which the present invention is directed, a melt reservoir is located at a position apparently apart from the alignment of crystalline substrates, and a melt is circulated t through gaps between adjacent substrates arranged vertically and then discharged from the gaps.
When the method using the alignment of crystalline substrates is compared with the sliding process, there is a common feature in using a sliding shutter mechanism for pouring and discharging a melt. However, the sliding shutter mechanism in the method using the alignment of crystalline substrates is principally different from that in the sliding process, since the sliding shutter mechanism is located at a position apart from the surfaces of the crystalline substrates and has a sliding direction unparallel to the surfaces of the crystalline substrates.
There is another fundamental matter for designing an apparatus to be used in the liquid-phase epitaxy process, which is how to completely remove a used melt from the surface of a crystalline substrate after the crystal growth. If the used melt remains on the surface of the crystalline substrate after the completion of crystal growth, the residual used melt would causes abnormal crystal growth or the formation of crystallographic defects.
In the sliding process, it is important how to design a distance in the order of .mu.m's from the surface of a crystalline substrate to a holder which is carried in contact with the surface of the crystalline substrate. The distance from the surface of the crystalline substrate to the holder, predetermined according to carrying speed, temperature for crystal growth, the kind of melt, etc., is generally held at a proper value within the range of 0 to 200 .mu.m.
On the other hand, in the method using the alignment of crystalline substrates, it is important to hold a gap the order of mm's order between the surface of one crystalline substrate and the surface of an adjacent substrate. The gap between the surfaces of the adjacent substrates is generally held at a proper value within the range of 2 to 4 mm, and 20 or more plates of crystalline substrates are arranged in said space. The said value is determined according to various conditions such as the coagulative force and surface tension of the melt at a temperature for crystal growth, and the adhesive force of the melt to the surface of the crystalline substrate. The gap in the method using the alignment of crystalline substrates is extremely different from the distance in the sliding process, since these methods are based on physical phenomena which has each technologically quite different from the other.
An object of the present invention is to overcome the problems in the above-mentioned conventional method using the alignment of crystalline substrates.
Another object of the present invention is to provide a new liquid-phase epitaxy method and a manufacturing apparatus therefor.